[New Rule] AWS security agent detection

### Description

Hello, I'd like to propose a detection that buils on this https://blog.richardfan.xyz/2026/03/14/pentesting-a-pentest-agent-heres-what-ive-found-in-aws-security-agent.html#can-i-protect-my-website-from-being-pentested-by-ai-agents

The detection as suggested in based on the user agent `securityagent` and it could target WAF logs but also webserver logs.

`user_agent.original : "*securityagent*"`

### Target Ruleset

None

### Target Rule Type

None

### Tested ECS Version

_No response_

### Query

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### New fields required in ECS/data sources for this rule?

_No response_

### Related issues or PRs

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### References

https://blog.richardfan.xyz/2026/03/14/pentesting-a-pentest-agent-heres-what-ive-found-in-aws-security-agent.html#can-i-protect-my-website-from-being-pentested-by-ai-agents

### Redacted Example Data

_No response_